This . . . oh wow. Given that you can choose to change your target as each attack resolves, this ability is pretty weird. You should know whether you dropped a foe after the second hit. If the foe doesn't drop, but would drop if you changed targets (thereby limiting the number of hits on that target to 2 and adding extra damage) . . . Ugh, my head hurts. I guess you're supposed to apply the damage, then retroactively remove it and add it to the third hit if the foe didn't drop on the second hit? *headdesk* Was this really necessary?
Worse than that, the bonus damage is always added to the last attack made against the target in the round. Basically, for each successful attack, you roll the jabbing style damage, then on the next successful attack, retract the jabbing style damage and reroll it.
Here's a weird example. Say you're fighting a goblin with Roll With It. On your second hit, the goblin uses Roll With It and fails its reflex save. You hit it with a third attack, but after retracting the damage from the second attack, you find that the goblin would have succeeded on his reflex save, and so it rolls out of range, taking no damage from the second hit and being out of range for the third.
Not sure how this is meant to be resolved. I guess the developer looked at hammer the gap and thought "How can we make this complicated enough monks will want it?"